Fix data race in X86FloatingPoint.cpp ASSERT_SORTED

[inglorion]

Summary

ASSERT_SORTED checks if a table is sorted, and uses a boolean to
prevent the check from being run again if it was earlier determined
that the table is in fact sorted. Unsynchronized reads and writes of
that boolean triggered ThreadSanitizer's data race detection. This
change rewrites the code to use std::atomic<bool> instead.

Fixes PR36922.

[rnk]

lgtm

llvm/trunk/lib/Target/X86/X86FloatingPoint.cpp

static int Lookup(ArrayRef<TableEntry> Table, unsigned Opcode) {
   if (I != Table.end() && I->from == Opcode)
     return I->to;
   return -1;
 }
 
 #ifdef NDEBUG
 #define ASSERT_SORTED(TABLE)
 #else
-#define ASSERT_SORTED(TABLE)                                              \
-  { static bool TABLE##Checked = false;                                   \
-    if (!TABLE##Checked) {                                                \
-       assert(std::is_sorted(std::begin(TABLE), std::end(TABLE)) &&       \
-              "All lookup tables must be sorted for efficient access!");  \
-       TABLE##Checked = true;                                             \
-    }                                                                     \
+#define ASSERT_SORTED(TABLE)                                                   \
+  {                                                                            \
+    static std::atomic<bool> TABLE##Checked(false);                            \
+    if (!TABLE##Checked.load(std::memory_order_relaxed)) {                     \
+      assert(std::is_sorted(std::begin(TABLE), std::end(TABLE)) &&             \
+             "All lookup tables must be sorted for efficient access!");        \
+      TABLE##Checked.store(true, std::memory_order_relaxed);                   \
+    }                                                                          \
   }
 #endif
 
 //===----------------------------------------------------------------------===//
 // Register File -> Register Stack Mapping Methods
 //===----------------------------------------------------------------------===//
 
 // OpcodeTable - Sorted map of register instructions to their stack version.